Not Applicable
Not Applicable
Not Applicable
This invention relates to electro ceramic components such as MEMS arrays and methods for fabricating electro ceramic components that can tolerate higher actuation voltages and operate with higher efficiency. Components are MEMS arrays or other micromachined elements.
Conventional MEMS array structures comprise Silicon on Insulator (SOI) array structures on which is fabricated an integrated electrode array. One of the problems encountered is voltage breakdown between electrostatically actuated elements and surrounding structures. The problem is evidently related to control of the electrostatic fields associated with high potential differences over short distances in the cavity structure and between the top surface of the electrostatic actuation electrodes and the bottom surface of the MEMS actuation element (hereafter referred to as the air gap) due to limitations of minimum wafer thickness of an SOI wafer where the air gap is nominally set by wafer thickness.
Typical structures have a conductive handle which is electrically connected to the thin actuatable layer. When these devices are connected to a potential, the fringing fields are forced beneath the mirror, reducing the available electrostatic force for a given amount of voltage. This necessitates a design wherein the mirrors have torsional hinge elements with greater compliance, resulting in slower devices with lower yield.
What is needed is a solution that preserves integrity of the structure of the actuatable element and allows an increase in the effective voltage without the danger of electrical breakdown.
According to the invention, a structure of a hybrid Micro Electro Mechanical System (MEMS) is provided wherein a plate comprises a thin actuatable layer of conductive silicon, such as a MEMS actuatable element, and a thicker handle layer of conductive silicon to provide structural integrity which are separated by a thin oxide, together forming an SOI wafer. This plate is mounted to a substrate, typically a ceramic, with the thin actuatable layer facing the substrate and separated by an airgap that is formed by creating, on the substrate, standoffs which come in contact with the plate. A suitable dielectric material useful as a standoff on the substrate is a photoresist that permits high aspect ratios (such as a 5:1 height to width ratio). Separation may be effected by other materials and deposition methods, such as by a screen printed adhesive, an electroformed standoff or glass beads. The plate may be attached using a mechanical fastening technique that permits heterogeneous expansion, as for example through a form of rivet. The plate of this structure, typically formed of Silicon on Insulator is referred to as a xe2x80x9cFlipped SOIxe2x80x9d because the handle is not mounted on the substrate, which is typical of these devices. Instead, the handle is unmounted, and the support is by means of standoffs.
This structure solves two problems: The first problem is potential voltage breakdown. The present structure mitigates this problem for a certain class of MEMS. In the particular case of a double-gimbaled mirror wherein the actuation force is created between the mirror element and the juxtaposed electrodes formed in the substrate, the applied voltage and resultant force can be boosted because the fringing fields can now spread onto the standoff (where the standoff is a dielectric or conductive material of a high resistivity). The advantage gained can be significant, allowing the device to be subjected to nearly double the electrostatic force for a given voltage as compared to conventional structures. The force enhancement effect of this type of a stand-off is configuration dependent, based on the height of the airgap, the overall size of the actuatable element, and the design of the double-gimbaled mirror. The fringing field problem, which increases with the aspect ratio of the air gap to the size of the device, is mitigated by this invention, permitting larger airgaps, greater tilt angle, faster displacement for a given voltage and higher breakdown voltage for a given gap.
The second problem solved is control of the airgap. The airgap of the actuatable element is now controlled by a separate deposition or electro-forming process. This decouples the airgap height from the handle thickness. Handle thickness is constrained to be within a narrow range. A handle thickness that is greater than or less than this range causes difficulties in fabrication and handling of the device due to its fragility.
However, the standoff according to the invention introduces a potential problem of susceptibility to fringing fields that may penetrate through them, causing crosstalk to adjacent MEMS elements. To mitigate this crosstalk problem, the substrate is coated with a highly resistive conductive material.
A further problem that arises by using this technique is vignetting of light for cases where light is obstructed by the handles of the flipped SOI structure. This problem is mitigated by construction of the handle with chamfered or terraced walls